The problem is because you are calling for FBSDKAccessToken.currentAccessToken()
before having called
FBSDKApplicationDelegate.sharedInstance().application(application, didFinishLaunchingWithOptions: launchOptions)
You can check for the access token anytime after calling the above line.
EDIT: Explanation
The above line lets the Facebook SDK process the launchOptions and extract the necessary information which it will require to recognise and persist the user for the application.
In cases where the user is already logged in, this simply initialises the Facebook SDK which in turn logs in the user on the basis of persisted data.